The invention relates to a jaw structure for alternating current instruments of the type commonly used as clamp-on, or hook-on voltmeters and ammeters that sense the magnetic field strength around a current carrying conductor, and more particularly, the invention relates to a shrouding means for the ends of such instrument jaws to protect an operator handling the instrument from the hazard of electrical shock that might be transmitted through the jaws to the instrument, if the iron material in the jaws were permitted to come in contact with a high voltage electrical conductor.
Heretofore, it has been common practice to provide hand-held, clamp-on ammeters and voltmeters with relatively movable soft iron jaws that are either completely uninsulated or that are only insulated completely from an encircled high voltage conductor when the jaws of the instrument are in their closed position. An example of the uninsulated prior art type of clamp-on instrument jaws to which reference is made is shown in U.S. Pat. No. 3,168,698 which issued on Feb. 2, 1965. Such uninsulated clamp-on jaw arrangements are relatively safe for the low voltage applications normally anticipated for the type of hand-held instrument described in that patent. However, for current and voltage monitoring clamp-on type instrument adapted for use in monitoring higher voltage conductors, it is more common to use clamp-on jaw structures that define a substantially insulated conductive loop when the jaws are positioned in their operating, or closed position. An example of such a pair of partially insulated jaws is shown in U.S. Pat. No. 3,611,136 which issued on Oct. 5, 1971. The type of insulated jaw arrangement shown in this latter patent provides considerable protection to the operator of a hand-held instrument because it is only possible for an electrical shock to be transmitted from a high voltage conductor to the jaws of the instruments when the jaws are in their open position. Moreover, since only one of the ends of the jaws of the instruments is completely exposed to contact with such a conductor when the jaws are in their open position, the risk of such a shock hazard is at least somewhat reduced. Still, it has been recognized that it would be possible for the one exposed jaw end on such an instrument to come into contact with a high voltage conductor and thereby possibly induce a harmful high voltage into the instrument and thence to an operator.
While the potential hazard of a high voltage shock inherent in prior art type uninsulated and partially insulated clamp-on jaw structures has long been recognized, a satisfactory alternative jaw structure has not heretofore been developed so far as is known. Such a lag in development may be due, in part, to the need to accommodate any safety or shielding insulating structure to the basic operating requirement of the jaws that makes it necessary to avoid any shielding structure that might prevent proper transformer action from occurring in the loop defined by the jaws when they are moved to their closed, conductor encircling position. Of course, any such improper transformer action resulting from only partial closing of such jaws could result in large instrument errors.
Accordingly, it is a primary object of the present invention to provide a pair of relatively movable hook-on type jaws for a measuring instrument with shrouding means that effectively protect the conductive ends of the jaws from being engaged by a high voltage conductor when the jaws are in their open position during movement of the jaws toward or away from a conductor with which the instrument is being used.
Another object of the invention is to provide a hook-on type instrument with a pair of relatively movable insulated jaws having shrouding means at the respective outer ends thereof to prevent a high voltage conductor from engaging said outer ends.
Yet another object of the invention is to provide a pair of shrouded jaws, for a hook-on type instrument, with shrouding means that are characterized by incorporating cup-shaped recess structures that enable the shrouding means to be closely nested within one another when the jaws are closed around a high voltage conductor.
Additional objects and advantages of the invention will become apparent from the description of it that follows taken in conjunction with the accompanying drawings.